generate_derived_keys_for_table() did not work correctly in the case where
- it had a potential index on derived table
- however, TABLE::check_tmp_key() would disallow creation of this index 
  after looking at its future key parts (because of the key parts exceeding 
  max. index length)
- the code would leave a KEYUSE structure that refers to a non-existant index.
  Depending on further optimizer calculations, this could cause a crash.